All 150mm slot-in filter system are listed here. Minimal color cast for ND filters. Filter holders available for multiple bulbous lenses, secure design and quality built.
Feel free to check out the reviews done here by ClubSNAP Singapore's largest photographic forum member "Scintillation" and images thread. Review is based on 100mm, but the 150mm performance is similar to 100mm.
Photosphere SG conducts regular demos as well to show the how to use the filters, feel free to like our facebook page to get updated events.